This invention relates to an electron beam exposure apparatus which scans a semiconductor wafer or the like with an electron beam for the purpose of pattern-writing, and more particularly to an apparatus of the kind described above in which dynamic focus correction can be easily carried out.
An electron beam exposure apparatus directs an electron beam toward and onto a workpiece such as a semiconductor wafer to write a desired pattern on the workpiece, and, in such an apparatus, an objective lens is disposed in the path of the electron beam to finely focus the electron beam thereby exposing the workpiece to the finely focused electron beam.
In the electron beam exposure apparatus, the period of time required until, after a semiconductor wafer set on a workpiece stage is exposed to an electron beam carrying a predetermined pattern and is then taken out, the next wafer is re-set on the workpiece stage, that is, the so-called throughput should be as short as possible. As means for improving the throughput, it has recently been attempted to widen a deflection field of the electron beam on a wafer so as to reduce the frequency of movement of the workpiece stage. However, an increase in the amount of deflection of the electron beam results in an increased degree of beam de-focus in proportion to the second power of the distance from the center to the position of deflection, and it becomes necessary to correct this beam de-focus.
The beam de-focus can be corrected by disposing a dynamic focus correction lens in the path of the electron beam and controlling the dynamic focus correction lens according to the position of deflection. Actually, however, a magnetic field produced by the dynamic focus correction lens intrudes partly into the magnetic path of the objective lens to cause such a disturbance that a constant magnetic field established by the objective lens changes gradually with time. As a result of the disturbance, such a new problem arises that the position of the deflected electron beam changes with time thereby degrading the positioning accuracy for the electron beam.
JP-A-60-140820 (Japanese Patent Un-examined Publication filed on July 25, 1985 assigned to HITACHI Ltd.,) discloses a method which corrects the beam de-focus due to the deflection of the electron beam and which can minimize degradation of the positioning accuracy for the electron beam as a result of the focus correction. JP-A-60-140820 teaches that, by selecting the diameter of the dynamic focus correction lens to be smaller than 1/2 of the inner diameter of the magnetic path, the undesirable disturbance by the magnetic field of the dynamic focus correction lens on the constant magnetic field of the objective lens can be reduced to less than 1/8 of the prior art value. However, it has been difficult with such a method to control position deviation of the electron beam to within the required range of accuracy. Therefore, in order to maintain the required accuracy, it has been necessary to wait the exposure for several ms which is the period of time required until the disturbance completely disappears, and it has been unable to expect a remarkable improvement of the throughput. For example, waiting for 5 ms at each time of beam deflection corresponds to a total waiting time of about 1 hour in the case of a 4-inch wafer.